Today's automotive safety standards require automotive vehicles to be equipped with seat belt reminder (SBR) systems for reminding a vehicle passenger to fasten the seat belt associated to the occupied vehicle seat. While until now, those seat belt reminder systems were mainly associated with front seats of the vehicle, future standards will require also rear seat to be equipped with such seat belt reminder systems.
Seat belt reminder systems typically comprise a seat occupancy sensor associated with a vehicle seat for detecting a presence of an occupant on the respective seat and for generating a signal indicative of such presence, and a seat belt fastening detector for determining a seat belt usage condition and for generating a signal indicative thereof. A control unit then uses the signals of the seat occupancy sensor and the seat belt fastening detector in order to determine whether the actual seat occupancy would require a non-fastened seat belt to be fastened and, if this is the case, to issue a corresponding warning signal.
The seat occupancy sensors usually comprise pressure-sensing devices integrated in the respective passenger seat for detecting a pressure induced by the presence of a passenger into the seat. The pressure-sensing devices, as e.g. disclosed in DE 42 37 072 C1, comprise a plurality of individual force sensors, which are connected in a suitable manner to a control unit designed for measuring a pressure-depending electrical property of said individual pressure sensors. These occupancy sensors have proven to be very reliable and well adapted to the detection of seat occupancy.
The seat belt fastening detectors typically comprise mechanical or magnetic buckle switches for detecting whether a latch of the seat belt is inserted into the seat belt buckle. One such buckle switch is e.g. disclosed in U.S. Pat. No. 5,871,063 A. As an alternative to the buckle switches, seat belt fastening detectors have been proposed which generate a buckled/unbuckled signal based on the tension in the seat belt.
One drawback of the known seat belt reminder systems lies in the fact that the seat occupancy sensor and the seat belt fastening detector usually have to be physically connected to the control unit by means of connection wires in order to be functional. This need for physically connecting the sensing device to the control unit however causes problems especially in modern cars equipped with a flexible seating system with removable and/or displaceable back seats.
In order to overcome this drawback, seat occupancy sensor systems have been proposed in which a signal representing the occupancy status is wirelessly transmitted from the seat to a control unit which is mounted in the vehicle. Such systems have e.g. been disclosed in documents U.S. Pat. No. 7,536,920 B2 or U.S. Pat. No. 7,639,125 B2. These systems solve the problem of the physical connection of the seat occupancy sensor or the seat belt fastening detector to the control unit. However, these systems suffer from the problem that the control unit, which receives a signal from a wireless seat sensor system, does not necessarily know in which location this seat is mounted in the vehicle, or whether it is correctly mounted at all. The system could, for instance, receive a signal from a seat stored in the trunk of the vehicle. Likewise, the absence of a signal from the seat sensor may be caused by a failure of the seat sensor system, or simply by the seat not being mounted in the vehicle.